In production of a semiconductor integrated circuit, an exposure apparatus has been widely utilized which transfers a fine circuit pattern drawn on a mask original plate on a wafer. Along with high integration and high functionality of an integrated circuit, an integrated circuit becomes finer. Thus, an exposure apparatus is required to form an image of a circuit pattern with high resolution on a wafer with a long focal depth, and shortening of the wavelength of the exposure light source is being advanced. The exposure light source has been shifted from conventional g-line (wavelength: 436 nm) or i-line (wavelength: 365 nm), to a KrF excimer laser (wavelength: 248 nm) or an ArF excimer laser (wavelength: 193 nm).
A synthetic quartz glass is excellent in transparency to light over a wide range from a near infrared region to an ultraviolet region, it has an extremely small thermal expansion coefficient and it is relatively readily processed. From such reasons, a synthetic quartz glass has been mainly employed as a photomask substrate for an exposure apparatus employing a light having a wavelength of from 170 to 400 nm as a light source.
A photomask substrate which is practically used at present may be one for an ArF excimer laser. As its specifications, it is resistant to an ArF excimer laser and in addition, approximately, the flatness on the surface is 0.5 μm on the entire photomask face, the degree of parallelization is at a level of 5 μm, and the birefringence is at a level of from 4 to 10 nm/cm.
JP-A-2003-515192 and JP-A-2001-302275 disclose a photomask which brings about a birefringence of at most 2 nm/cm.